Air conditioning cuts off after engine warms up

Hello,
I have a 2000 Ford Escort with an air conditioner that cuts off after about 10 minutes of driving. I have taken it to an Exxon Service center once and a Ford Service center (here in Northern Virginia)
several times, where they said they couldn't help me because the air conditioner always works for them when they're examining it and they can't locate the problem while it's working.
I have even stopped in at the Ford place unannounced when my a/c cut off on nearby I-95 (which it always does) and they've immediately taken a look at it -- but before they can narrow down the problem, the air conditioner cuts back on, and they shrug their shoulders and say they can do nothing. (The a/c often cuts back on for a minute or two when I stop the car at a red light and such. In fact, when I'm driving on I-95, the a/c will begin working for a brief time every time I stop at a toll booth, even though it stays off for the rest of the trip.)
This problem began two years ago. I was psychologically prepared at that time to be told that it would be a very expensive repair -- in fact, I expected that -- but I was not expecting to find out that no one could even TELL ME what was wrong with the a/c. Of course, over the past year, many apparent "professionals" have, told me what they thought was wrong with it, but most of them say very different things from one another.
So my question is: Where can I take this car (hopefully in the mid-Atlantic region) to at least get it definitively diagnosed, if not repaired?
I'm tired of taking it in for one-day look-overs just to be told that the problem cannot be duplicated at the service station, and therefore they can do nothing for me.
So to repeat: Where can I take this car (hopefully in the mid-Atlantic region) to at least get it definitively diagnosed, if not repaired?
Thanks for your help.
PS I was going to finish with a paragraph or two about my new theory: that the poor "diagnosibility" of car a/c's is a scam designed to keep service departments busy, but I've decided not to, since my main concern for now is getting the thing diagnosed -- and perhaps even fixed :)
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I'm in Northern VA also. E-mail me privately, and I'll try to help you.
Thanks,
CJB

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